In the growth condition(s) of plants, numerous secondary metabolites (SMs) are produced by them to serve variety of cellular functions essential for physiological processes, and recent increasing evidences have implicated stress and defense response signaling in their production. The type and concentration(s) of secondary molecule(s) produced by a plant are determined by the species, genotype, physiology, developmental stage and environmental factors during growth. This suggests the physiological adaptive responses employed by various plant taxonomic groups in coping with the stress and defensive stimuli. The past recent decades had witnessed renewed interest to study abiotic factors that influence secondary metabolism during in vitro and in vivo growth of plants. Application of molecular biology tools and techniques are facilitating understanding the signaling processes and pathways involved in the SMs production at subcellular, cellular, organ and whole plant systems during in vivo and in vitro growth, with application in metabolic engineering of biosynthetic pathways intermediates.

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Stress and defense responses in plant secondary metabolites production

Improving production of plant secondary metabolites through biotic and abiotic elicitation

Higher plants undergo a variety of stresses and to combat those stresses they acclimatize themselves by producing diverse secondary metabolites. These secondary metabolites also have a wide range of industrial applications and hence they serve as candidates for commercialization. Owing to the constraints faced by natural plant extraction, plant cell/tissue culture has emerged as an alternative platform for the in vitro production of value added bioactive secondary metabolites. Implementation of several productivity enhancement strategies, including elicitation, can overcome the limitations faced by plant cell technology that hampers its extensive commercialization. Elicitation is a technique that involves exogenous addition of elicitors (abiotic or biotic) in the growth medium which consequently triggers stress response with concomitant enhancement in secondary metabolite production. Elicitor induced stress results in the activation of several defense-related genes or inactivation of non-defense-related genes, transient phosphorylation/dephosphorylation of proteins, expression of enzymes whose information can be used to ascertain the biosynthetic pathways of many secondary metabolites. Furthermore, integration of transcriptomics, proteomics and metabolomics with system biology can aid in discovery of novel genes, transcriptional factors and several biosynthetic pathways which in turn can serve as a valuable tool for metabolic engineering and gene manipulation for enhancing the yield and productivity of secondary metabolites.

Elicitation: a stimulation of stress in in vitro plant cell/tissue cultures for enhancement of secondary metabolite production

Plant secondary biosynthetic pathways are exceedingly inducible by elicitors and facilitate enhanced metabolite production using plant cell tissue and organ cultures. Elicitors can regulate large number of control points and trigger the expression of key genes with increased cellular activities at biochemical and molecular level involving signal compounds. A large number of chemical elicitors viz: jasmonic acid (JA), methyl jasmonate (MeJA), salicylic acid (SA), acetyl salicylic acid (ASA), ethylene (ET) and ethrel (Ethe), heavy metals (HM), many types of chemical compounds (natural or synthetic) and their combinations are used for elicitation studies. Cell suspensions and hairy roots are commonly used culture systems followed by adventitious roots and multiple shoots for elicitation experiments. Amongst the elicitors and concentrations used 100 µM MeJA was found optimum for secondary metabolite enhancement in majority of experiments compared to SA and JA. Elicitor treatments promoted yield enhancements starting from 1.0 to maximum of 2230-fold across plant species studied. Elicitors singly with media additives, combination of elicitors and elicitors other than signal compounds along with hormones were found beneficial for enhanced secondary metabolite production. Further, a combination of target gene over expression and elicitor treatment also supported higher secondary product yield. The present communication presents information exclusively about the use of chemical elicitors for secondary metabolites production in vitro covering approximately more than a decade of research at one place in one review. Further, this extensive appraisal will be useful for the understanding and manipulation of secondary metabolites for enhanced production in vitro.

Chemical elicitors versus secondary metabolite production in vitro using plant cell, tissue and organ cultures: recent trends and a sky eye view appraisal

Essential oils (EOs) are a mixture of natural, volatile, and aromatic compounds obtained from plants. In recent years, several studies have shown that some of their benefits can be attributed to their antimicrobial, antioxidant, anti-inflammatory, and also immunomodulatory properties. Therefore, EOs have been proposed as a natural alternative to antibiotics or for use in combination with antibiotics against multidrug-resistant bacteria in animal feed and food preservation. Most of the results come from in vitro and in vivo studies; however, very little is known about their use in clinical studies. A systematic and comprehensive literature search was conducted in PubMed, Embase®, and Scopus from December 2014 to April 2019 using different combinations of the following keywords: essential oils, volatile oils, antimicrobial, antioxidant, immunomodulation, and microbiota. Some EOs have demonstrated their efficacy against several foodborne pathogens in vitro and model food systems; namely, the inhibition of S. aureus, V. cholerae, and C. albicans has been observed. EOs have shown remarkable antioxidant activities when used at a dose range of 0.01 to 10 mg/mL in cell models, which can be attributed to their richness in phenolic compounds. Moreover, selected EOs exhibit immunomodulatory activities that have been mainly attributed to their ability to modify the secretion of cytokines.

https://www.mdpi.com/2072-6643/11/11/2786/pdf

Antimicrobial, Antioxidant, and Immunomodulatory Properties of Essential Oils: A Systematic Review

Enhanced bacoside production in shoot cultures of Bacopa monnieri under the influence of abiotic elicitors

Bio-active compounds, isolated from medicinal plants play a vital role in modern medicine as some of them have become very potent drugs. Although these compounds are produced and accumulated in different parts of the plants, there are many bottlenecks in getting plant-based bio-active compounds. In particular, phytocompounds are limited to a particular species/genus and are produced only under specific conditions, such as pathogen attack, stage of growth and development, etc. In this regard, metabolic engineering is very promising as it offers the possibilities of overcoming the dearth of desired plant compounds by using various strategies that include increased flux of precursors, blocking of competitive pathway by using the intermediate compounds, introducing new metabolic pathways, overcoming rate-limiting steps, and the overexpression of regulatory genes or transcription factors for inducing the biosynthetic pathways. The metabolic engineering approach has been exploited in transforming plants as “cell factories” for producing various bio-active compounds. Due to the tremendous biological potential of these compounds, the understanding of their synthesis, accumulation, and manipulation in different parts of the plant along with their regulation is very crucial. By utilizing different genomics and metabolomics tools, the production of various bio-active compounds has been enhanced. This chapter provides the present-day knowledge on the production of some plant-derived bio-active compounds, such as polyphenols, alkaloids, terpenes, and saponins. Further, various biotechnological approaches for increasing the accumulation of bio-active compounds through metabolic pathways engineering in plants are discussed in this chapter.

Metabolic Engineering Strategies for Enhancing the Production of Bio-active Compounds from Medicinal Plants

Saffron is considered to be the costliest spice of the world. It has been regarded as highly valued medicinal plant in Ayurveda to treat various ailments. Over the past few years, considerable interest has developed in saffron because of its anticancer, antimutagenic, antioxidant and immunomodulatory properties. Saffron's colour, bitter taste and aroma are its three main and peculiar characteristics, which are conferred by three chemicals namely: crocin, picrocrocin and safranal, respectively. The present review focuses on recent research/progress made in saffron in the area of functional genomics and highlights the potential of several genes and transcription factors involved in carotenoid/apocarotenoid pathway and responsible for flavour and aroma of saffron.

Functional genomics of apocarotenoids in saffron: insights from chemistry, molecular biology and therapeutic applications.

Shoot cultures of Bacopa monnieri L. (Brahmi) were cultivated in shake flask (1 L) and two bioreactor systems: Growtek® (1 L) and modified bench top air lift bioreactor (ALB) (5 L). Continuous immersion and aeration supported excellent growth and enhanced levels of bacoside (A3+A2) production in ALB. Growth index (GI) in terms of dry wt was recorded maximum in the ALB (5.84), followed by Growtek® (4.22) and shake flasks (2.61) after 4 wk of incubation. ALB recorded the highest number of shoots (443.33), as compared to that in Growtek® (42.67) and shake flasks (23.33). Furthermore, bacoside production in shoot cultures of the ALB system was ~ 1.75-fold higher as compared to cultures in shake flask. Nutrient exhaustion in spent medium from the various systems was also measured. Present work reports the feasibility of shoot cultivation in liquid medium to develop a suitable bioreactor strategy.

http://nopr.niscair.res.in/bitstream/123456789/34006/1/IJBT%2014%284%29%20547-551.pdf

Bacoside biosynthesis during in vitro shoot multiplication in Bacopa monnieri (L.) Wettst. grown in Growtek and air lift bioreactor

A study was conducted to evaluate bacoside (expressed as sum of bacoside A3 and A2) production on a monthly basis in net house grown plants of Bacopa monnieri (accession BM001) collected from Jammu region. Bacoside content differed during the course of the whole year. The whole herb is used commercially for extracting the memory enhancing molecule bacosides from the plant. The study significantly points out that the maximum bacoside yield was obtained during August to October (monsoon period) which also coincides with the favourable period for plant growth and would be suitable for plant harvest. Amongst the various individual plant parts, the highest content was recorded in the leaves (6.06 mg g DW) followed by stem (5.13 mg g DW) and least in the roots (3.19 mg g DW).

Munish Sharma

Papers

Enhanced bacoside production in shoot cultures of Bacopa monnieri under the influence of abiotic elicitors

Metabolic Engineering Strategies for Enhancing the Production of Bio-active Compounds from Medicinal Plants

Functional genomics of apocarotenoids in saffron: insights from chemistry, molecular biology and therapeutic applications.

Bacoside biosynthesis during in vitro shoot multiplication in Bacopa monnieri (L.) Wettst. grown in Growtek and air lift bioreactor

Annual variation in bacoside content of bacopa monnieri (l.) wettst plants